February 25 2025 (in-person)

Ben Gilbert (Livermore)

Host: Spencer Klein

Title: Measurement of photonuclear dijet production in ultra-peripheral Pb+Pb collisions at the LHC with the ATLAS Detector

Abstract:  In ultra-relativistic heavy ion collisions, the charged ions produce an intense flux of equivalent photons. Photon-induced processes are the dominant interaction mechanism when the colliding nuclei have an impact parameter larger than the nuclear diameter. In these ultra-peripheral collisions (UPCs), the photon provides a clean, energetic probe of the partonic structure of the nucleus, analogous to deep inelastic scattering. This talk presents a measurement of jet production in UPCs performed with the ATLAS detector using high-statistics 2018 Pb+Pb data. Events are selected using requirements on jet production, rapidity gaps, and forward neutron emission to identify inclusive photo-nuclear hard-scattering processes. These measurements also include detailed studies of rapidity gap distributions and nuclear break-up effects, allowing for precise comparisons between data and theory for inclusive photo-nuclear processes. The measured cross-sections are compared to theoretical models in phase-space regions where significant nuclear PDF modifications are expected but not well constrained by world data, demonstrating the potential of these data to provide a strong new constraint on nPDF effects.

March 4 2025 (in-person)

Thomas Richardson (LBNL)

Host: Andre Walker-Loud

Title: Two-baryon interactions and currents from large-Nc QC

Abstract:  The large-Nc limit of QCD leads to an emergent spin-flavor symmetry in the baryon sector. This symmetry explains several properties such as the ratios of masses, magnetic moments, and meson couplings. It also provides an explanation of the relative sizes of couplings in phenomenological two-nucleon potentials. We discuss recent applications of the spin-flavor symmetry in large-N QCD to nuclear forces and currents relevant for beyond the Standard Model searches including neutrinoless double beta decay and CP violation. We also discuss new symmetries that emerge in the unitary limit for two-baryon scattering.